The invention relates to an exhaust-gas turbocharger turbine.
An exhaust-gas turbocharger turbine of the generic type is described in DE 42 32 400 C1. In this case, an adjustable flow guide device having guide blades for controlling the exhaust-gas passage in normal operation, in particular within the part-load range of the internal combustion engine, is located in the flow gap between the guide passage and the turbine impeller. In addition, the adjustable flow guide device, which is located continuously in the flow gap, is also used for a braking operation of the internal combustion engine.
A disadvantage in this case, however, is the fact that, although better acceleration is achieved within the part-load range by the flow guide device having the guide blades, efficiency losses and consumption disadvantages occur in full-load operation.
Similar adjustable low guide devices for exhaust-gas turbines of exhaust-gas turbochargers are described in DE 43 03 520 C1 and 43 03 521 C1.
With regard to the prior art, reference is also made to EP 0 571 205 B1 and U.S. Pat. No. 4,776,168, which each describe adjustable flow guide devices for controlling the degree of charging of the exhaust-gas turbine in normal operation, in which case use in engine braking operation is also partly envisaged.
The object of the resent invention is to design an exhaust-gas turbocharger turbine in such a way that a high braking performance in braking operation of the internal combustion engine is achieved, although no efficiency losses are to occur at full load. At the same time, however, the dynamic driving behaviour of the vehicle is to be improved.
Unlike the prior art, in which a guide cascade with mostly adjustable guide blades projected continuously in normal operation into the flow gap for controlling the exhaust gas flow rate, a fixed guide cascade is now used according to the invention, this guide cascade being mounted in normal operation in recesses of a housing wall of the turbine housing. This means that the flow gap is completely free in normal operation, for which reason efficiency losses no longer have to be tolerated during full-load operation.
In an inventive manner, with the guide cascade, the supercharging potential of the exhaust-gas turbocharger is now also utilized during the engine braking operation. To this end, during braking operation, the guide cascade is pushed with an appropriate axial displacement of the support into the flow gap, in which case the guide blades can be arranged radially so as to be optimized in terms of flow in such a way hat the exhaust-gas turbine continues to run even at a low residual-gas flow. On the one hand, this supercharges the engine, whereby the gas flow on the exhaust-gas side is then consequently increased in turn, and on the other hand the exhaust-gas back pressure is thus also increased, which at the same time allows the turbine to rotate even faster.
This process is repeated until the exhaust-gas back pressure is so great that the engine speed no longer increases. The braking action of the engine and thus that of the vehicle takes place because the additional air and the additional exhaust-gas back pressure in the combustion space have a very powerful braking effect on the piston and, at the end of the compression phase, the compressed air is discharged into the exhaust system in a known manner via a valve additionally located in the cylinder head, e.g. a constant choke. In this way, the energy in the form of compressed air is not returned again to the piston.
With the solution according to the invention, a higher braking performance with simultaneous reduction in the thermal loading of engine components, in particular of injection nozzles, is achieved. At the same time, an improvement in the dynamic driving behaviour of the vehicle is thereby achieved, since the turbocharger continues to run during the braking of the engine and a charge pressure is immediately present again at the engine during a subsequent acceleration action. In this way, a so-called turbohole is avoided.
Apart from a small clearance, which is necessary for the axial displacement of the guide blades, a virtually closed housing wall, with at the same time a free flow gap, is achieved by the design of the housing wall part in the region of the guide blades in the form of a female piece in a mating profile shape for the guide blades. Thus no disturbing contours, which would impair the efficiency of the turbocharger, are located in the guide passage and the flow gap during normal turbocharger operation, i.e. in fired operation.
In an advantageous refinement of the invention, the braking device is designed as a prefabricated module and can be pushed as a unit into the turbine. In this case, at least the guide cascade with the guide blades, the support and the housing wall part, which forms the female piece, are parts of the module. Of course, further parts may if need be also be integrated and pushed as a prefabricated module into the exhaust-gas turbocharger turbine.
Advantageous refinements and developments of the invention follow from the subclaims and from the exemplary embodiments described in principle below with reference to the drawings.